1. Field of the Invention
The present invention relates to a reflector comprising a flexible sheet of reflective material stretched over a frame to define a curved reflecting surface. Particularly, but not exclusively, the invention relates to the shaping of a thin reflective film in the construction of a part spherical, spheroidal or toroidal mirror.
2. Related Art
Reflectors comprising a sheet of reflective material formed into part-spherical and other curved configurations are known. For instance, flight simulators and other optical training systems typically incorporate spherical mirrors which are conventionally produced by stretching a thin reflective film (typically a metallized plastics material) over a support frame which defines the necessary curvature, and then partially evacuating air from one side of the film.
In more detail, conventional methods for constructing such spherical mirrors comprise cutting a sheet of the reflective film into an appropriate shape and then stretching the film over the walls of a rigid support frame the edges of which have the appropriate curvature (i.e. lie on the desired curved surface). The edges of the film are sealed with respect to the frame so that the film and the frame together form a substantially air-tight chamber. The shaping of the mirror is then completed by partially evacuating the chamber so that the thin film assumes the desired spherical configuration, i.e. the film is sucked into the chamber forming a surface corresponding to a segment of a sphere. The same method can be used to construct mirrors with other curved geometries.
A problem with such methods of construction is that distortion can be introduced in regions towards the edges of the reflective film resulting from the need to support the film such that it has the necessary curvature in two orthogonal directions.
For instance, in the construction of spherical mirrors for use in the off-axis optical systems of flight simulators, the film of flexible material is initially cut into the shape of a blank for a frustum of a cone, having two relatively long side edges and two relatively short end edges. When mounting the film to its support frame, the relatively long edges are fixed to side walls of the frame first so that they are wrinkle free but without introducing significant tension into the film. The film will readily accept the curvature imposed by securing the two side edges of the film to the side walls of the mounting frame. Indeed the film is typically obtained from a roll and so will already have a tendency to curl in one direction.
The distortion referred to above is a result of the need to secure end edges of the film to end walls of the support frame, the edges of which curve in a direction substantially orthogonal to the direction of curvature of the edges of the side walls of the support frame. That is, initially (once the side edges of the film have been secured to the support frame) the end edges of the film will form a chord extending between the side walls of the support frame. To secure the end edges of the film to the support frame the film must be stretched onto its end walls. This stretching process can introduce areas of tension and/or looseness into the film which result in the distortion of the film at its edges once the chamber has been evacuated to produce the desired mirror shape. Clearly any such distortion will have an adverse effect on the performance of the mirror.
To obviate this problem, spherical mirrors are often made to a size larger than is actually required and the distorted regions are simply masked off or otherwise hidden.
An alternative solution to this problem is proposed in U.S. Pat. No. 4,592,717. This discloses apparatus comprising a support frame generally as described above but wherein the end edges of the film are mounted to the end walls of the support frame using flexible stretching members. The stretching members are initially straight (following the chords defined by the end edges of the film as mentioned above) but are subsequently bent to match the curvature of the edges of the end walls of the support frame. As the stretching members are bent they force the end edges of the reflective film onto the end walls of the frame to which they are then clamped. According to this U.S. patent, the configuration and positioning of the stretching members is such that as they are bent they stretch the film slightly beyond its yield point and in such a way as to eliminate the irregularities in tension that would otherwise cause distortion in the film.
In another known method for forming curved mirrors, a support frame substantially as described above is provided with extensions to its side walls which extend beyond the frame end walls and are closed off by secondary end walls. The film is cut larger than required and is initially mounted to the frame with its end edges secured to the secondary end walls so that the film extends beyond the frame end walls which define the desired size and curvature of the mirror. A curved former, having the same profile as the frame end walls, is then used to force (and thus stretch) the film onto the frame end walls and clamp it thereto. The extension sections can be either left in place or removed if desired.